1. Technical Field of the Invention
The present invention relates generally to an ejector pump in which a jet of a main fluid emitted from a nozzle serves to suck a sub-fluid, and more particularly to an improved structure of such an ejector pump which is suitable for use in a fuel cell system.
2. Background Art
In order to improve a utility factor of fuel and power generation efficiency of fuel cells, off-gas circulation systems are known which work to suck the off-gas discharged from hydrogen electrodes of the fuel cells using a pump and mix it with fuel to be supplied to the fuel cells. The pump used to circulate the off-gas is usually implemented by an ejector pump because it can be actuated using kinetic energy of the fuel and is of great advantage to power saving. For instance, Japanese Patent First Publication No. 2001-266922 teaches the above type of system.
Fuel cell systems equipped with the ejector pump, however, encounter the problem in that when the fuel cells suspend power generation suddenly, that is, when the fuel cells suspend consumption of a hydrogen gas suddenly, the ejector pump stops transforming a static pressure of the hydrogen gas stored within a fuel tank into a dynamic one, thereby causing the static pressure of the hydrogen gas outputted from the fuel tank to act on the fuel cells as it is through the ejector pump, which, in the worst case, results in breakage of electrode films of the fuel cells, thus leading to a failure in operation of the fuel cells. Moreover, repeated application of the pressure of the hydrogen gas to the electrode films will precipitate the fatigue of the electrode films undesirably. If the supply of the hydrogen gas is cut within the fuel tank, a residual pressure in a line between the fuel tank and the ejector pump is exerted on the fuel cells through the ejector pump, which gives rise to the same problem as described above.
The above type of fuel cell systems also have an additional problem in that when the amount of hydrogen gas supplied to the fuel cells is insufficient, the dynamic energy required to circulate the off-gas will be insufficient, thus resulting in lack in a circulated amount of the off-gas.